An individual gravimeter can be used to measure gravity in a local area. A pair of gravimeters can be used cooperatively to detect a differential gravity between two locations. Multiple differential gravimeters can be used to develop a three-dimensional map of gravity across a field or other region. Such 3-D mapping has been proposed in order to monitor fluid flow in-situ in subterranean reservoirs, such as oil fields.
A gravimeter must be extremely sensitive. For example, sensitivity below 1 micro-Galileo is necessary in many applications. Such extreme sensitivity, however, requires very high immunity to noise sources. Error can be introduced into the output signal of a gravimeter from noise sources such as electromagnetic interference, horizontal components in the acceleration of a free-falling mass, mechanical misalignment of sub-components, mechanical shock, and Coriolis forces that arise due to the rotation of the Earth.
Gravimeters have been developed that are based on the principle of balancing the weight of a fixed mass with forces from a normal or superconducting spring. Gravimeters such as these, however, can be difficult to setup and calibrate. In addition, such gravimeters can be sensitive to environmental influences such as temperature or vibration.
Gravimeters based on the measurement of the motion of a falling mass have also been developed. A sensor system is used to monitor the acceleration of the falling mass during its free-fall. In some instances, such a gravimeter utilizes a mechanical carriage to lift a test mass to a position from which it can subsequently free-fall downward. Typically, these gravimeters utilize a vacuum chamber to eliminate effects of air resistance on the acceleration of the test mass. There are several disadvantages associated with these gravimeter systems, however. The size and complexity of the mechanical carriage used to position the test mass typically limits the compactness of the system. In turn, the use of a mechanical carriage commonly requires that the vacuum chamber be quite large. As a result, the cost and expense associated with using such free-fall gravimeters precludes their use in many applications.
To overcome some of the drawbacks associated with the use of a mechanical carriage, gravimeters that utilize a piezoelectric launcher to vertically launch a test mass upward have been developed. The test mass is launched so that it begins a free-fall downward once it reaches the apex of its flight. In addition to some of the drawbacks of other prior-art gravimeters, however, the sensitivity of these gravimeters is limited due to shock and vibration associated with the piezoelectric launcher itself. This mechanical energy manifests itself as noise into the output signal, thereby reducing signal-to-noise ratio and sensitivity of the gravimeter.
There exists a need, therefore, for a gravimeter that avoids or mitigates some or all of the problems associated with prior-art gravimeters.